Uranium is produced from uranium-bearing ores by various procedures which employ a carbonate or acid lixiviant to leach the uranium from its accompanying gangue material. The acid lixiviants usually are formulated with sulfuric acid which solubalizes uranium as complex uranyl sulfate anions. The sulfuric acid normally is used in a concentration to maintain a pH between about 0.5 to 2.0. However, mild acidic solutions such as carbonic acid solutions, having a pH between about 5.0 and 7.0 may also be employed. Carbonate lixiviants contain carbonates, bicarbonates or mixtures thereof which function to complex the uranium in the form of water-soluble uranyl carbonate ions. The carbonate lixiviants may be formulated by the addition of alkali metal carbonates and/or bicarbonates or by the addition of carbon dioxide either alone or with an alkyline agent such as ammonia or sodium hydroxide in order to control the pH. The pH of the carbonate lixiviants may range from about 5 to about 10. The carbonate lixiviants may also contain a sulfate leaching agent. The lixiviant also contains a suitable oxidizing agent such as oxygen or hydrogen peroxide.
The leaching operation may be carried out in conjunction with surface milling operations wherein uranium ore obtained by mining is crushed and blended prior to leaching, heap leaching of all piles at the surface of the earth, or in-situ leaching wherein the lixiviant is introduced into a subterranean ore deposit and then withdrawn to the surface. Regardless of the leaching operation employed, the pregnant lixiviant is then treated in order to recover the uranium therefrom. One conventional uranium recovery process involves passing the pregnant lixiviant through an anionic ion exchange resin and elution of the resin with a suitable eluant to desorb the uranium from the resin. The resulting concentrated eluate is then treated to recover the uranium values, for example, by precipitating uranium therefrom to produce the familiar yellowcake.
The anionic ion exchange resins employed for uranium concentration are characterized by fixed cationic adsorption sites in which the mobile anion, typically chloride or another halide, hydroxide, carbonate or bicarbonate, is exchanged by the uranyl complex anion. Such anionic ion exchange resins are disclosed, for example, in Merritt, R. C., THE EXTRACTIVE METALLURGY OF URANIUM, Colorado School of Mines Research Institute, 1971, pp. 138-147, which are hereby incorporated by reference. Suitable anionic ion exchange resins may take the form of polymers or copolymers of styrene substituted with quaternary ammonium groups or polymers or copolymers of pyridine which are quaternized to form pyridinium groups.
In many areas where a leach operation, such as described above, is applicable, contaminants such as molybdenum values are also present in the ore. Since the molybdenum content in the leachate is generally the highest among the impurities, much of the efforts have been directed to the removal of molybdenum from process streams. The molybdenum will react similarly to the uranium in that the molybdenum values will oxidize and will leach into the solution along with the uranium. Likewise, the molybdenum values will be adsorbed onto the ion-exchange column and are eluted from the resin with the uranium values by the eluant. When the pregnant eluate is subjected to steam stripping, a portion of the molybdenum values, e.g., molybdic acid, will co-precipitate with the uranium values and is considered an undesirable contaminant in the yellowcake. If the molybdenum content in the yellowcake exceeds a specified value, e.g., 0.6 percent by weight, the yellowcake may require further extensive processing before it will be accepted by a commercial refiner.
It is also recognized that the presence of molybdenum in the pregnant lixiviant tends to reduce adsorption of uranium by the anionic ion exchange resin. Merritt discloses at pages 154, 163, and 164 that the presence of molybdenum in the pregnant lixiviant tends to poison the ion exchange resin, thus reducing the adsorption of uranium by the resin which results in decreased resin loading. Accordingly, the need for reducing the amount of molybdenum in the pregnant leachate prior to passing the leachate over an ion exchange resin is readily apparent.
A number of chemical or procedural techniques have been described to remove or at least decrease the molybdenum content of various process liquors. One technique teaches the addition of Na.sub.2 S or NaHS to precipitate molybdenum and other heavy metals. However, the sulfides will also be consumed in reduction reactions with these other metals. Excess sulfide ion will cause incomplete precipitation and may also be adsorbed by ion exchange resin and then decomposed to form elemental sulfur.
Another technique for molybdenum removal is co-precipitation of molybdenum and ferric ion by partial neutralization of the acidic leachate with lime. The effectiveness of the technique is profoundly affected by the solution pH. As the pH increases, the molybdenum removal improved. However, the uranium loss, either occluded or co-precipitated with the iron/molybdenum product, increases to prohibitive levels above pH 3.4. When the pH of the leachate from a uranium leaching process is between 6 and 8, it would be impractical to adjust the pH to below 3.4 by use of acid due to several factors such as the cost of acid, the possible loss of carbonate, corrosion of the equipment, and contamination of the circuit with more ions. Thus it seems that to obtain improved molybdenum precipitation by increasing the pH, i.e., above 3.4, prohibitive loss of uranium values must be accepted.
Additionally, the presence of silica along with molybdenum in the rather reducing condition seems to yield silica polymolybdate which is strongly distributed along the outer shell of the ion exchange resin. Attempts to satisfactorily regenerate such poisoned resin have failed. For example, upon regeneration with HCl solution, the resins turned into deep blue typical of silicomolybdate blue. While not wishing to be bound by the above chemical theorization, it is clear that it is desirable to remove both silica and molybdenum from the process stream. Of course, it is also desirable to remove any other contaminant ions if possible to reduce the ionic concentration of the leaching circuit.